Welcome innkeepper!
Your B&B sounds fascinating. I'm a big fan of B&Bs, and I'll certainly try to stay with you if I ever make it to Ottowa. Unfortunately, that might not be for a while, since I live in California.
I have a question, and I mean it completely seriously. Do you want to know if your inn is haunted, or do you want to believe that it is? Let me say this again, so I'm completely clear: I'm not trying to be clever. Either answer is acceptable. It just changes the kind of advice that I can give.
If you'd like to explore the belief that your inn is haunted, there's a huge array of technological devices and spiritual practices that you can employ: ouija boards, EVPs, EMF detectors, pendulums, dowsing rods, and K2 meters, to name a few. You could even contact a local paranormal society and have them investigate. I'm not an expert on those myself. I don't think that these techniques can give you objective evidence, though.
Let me take a break here and say, yet again, that I'm being serious. I'm not a believer in ghosts myself– I'm waiting for compelling evidence. Still, you have the absolute right to your own beliefs, and I will support that to the grave (and perhaps beyond)!
On the other hand, if you want to know, as best we can tell, if there are ghosts in your inn, then we need to start by eliminating mundane causes.
First, I'd start by having your local gas company check for high carbon monoxide levels. There's nothing really to indicate that, but at least down here they'll do it for free, so why not rule it out? Of the various things that could cause false positives, that's the only one I know of that's actually dangerous, so I've made it a rule to start by having people check.
Next, we want to gather as much information as we can about the incident. Can you find out the date and time? If there are incidents ongoing, it's probably a good idea to keep a journal. It's also important to make sure that you write down exactly– and only– what you saw, heard, and felt, and indicate the date, time, and weather conditions.
If you can find out the time of day that your sister-in-law saw the woman in mourning, then you might try reproducing the situation. I'd be interested in seeing a photograph taken of the mirror. If you want to take this route, here are the steps I'd take:
- Take the photograph at the same time of day that your sister-in-law had the experience
- Make sure that the flash is turned off
- Position the camera at the same height and facing the same direction as your sister-in-law's head.
If you could upload the photo to Flickr, ImageShack, or another photo hosting service, and post a link to it here, I would be interested in having a look.
I should offer a caveat– I'm an armchair analyst. Most of my experience comes from watching too much Ghost Hunters. Take my advice with whichever dose of salt you find appropriate.